For generating a high precision voltage reference, there are mainly three types of technologies conventionally used. The first type of technology utilizes a standard battery such as a Weston standard cell. Until recently, the standard battery has been used as a national voltage standard. The standard battery generates a DC voltage of 1.018636V at 20.degree. C. However, because the output voltage level is sensitive to temperature or mechanical vibration, the standard battery must be installed in a vibration free environment with a precise temperature control and management.
The second type of technology is a reference voltage generator using a zener diode (constant voltage diode). This technology is most widely used in voltage measurement instruments, voltage generators and the like. However, since zener diodes utilize an avalanche effect in the PN junction of the diodes, the resultant voltages involve relatively large short term variations. Zener diodes also involve long term voltage variations such as in the order of a few ppm (parts per million). Thus, zener diodes are sometimes not an ideal voltage reference when a higher level of voltage precision is required.
The third type of technology is a Josephson voltage standard utilizing a Josephson effect. In a Josephson junction, there is precise relationship between DC output voltages and frequencies of a microwave signal applied thereto. It is known in the art that frequency has a higher degree of accuracy than other units such as voltage, resistance, or the like. Thus, precision DC voltages corresponding to frequencies (F/V conversion) can be obtained by applying a high accuracy frequency to the Josephson junction. The resultant DC voltage has accuracy which is comparable to that of the frequency. However, the Josephson voltage standard is not ideal for general purpose apparatuses because this voltage standard apparatus is large and expensive due to the need of an array of super conductive Josephson junctions, liquid helium to cool the junction array, and a relatively large mechanical structure since it involves a millimeter wave technology.
As explained above, the conventional reference voltage generators have disadvantages in applying to small sized apparatuses such as test instruments. Thus, there exists practical difficulties in such instruments to have a reference voltage generator for generating high precision DC voltages such as in the order of 0.1 ppm.